Semiconductor contaminant sensing system and method

ABSTRACT

Contamination of a fluid medium is measured using a thin-wafer sensor having a front surface and a back surface. The thin-wafer sensor is made of relatively pure p-type silicon and has a thickness less than the bulk diffusion length of the material. The back surface is placed in physical contact with the fluid being monitored either by building the sensor into a fluid testing chamber in line with the system using the fluid, or by immersing an encapsulated version of the sensor into the fluid medium. A photovoltaic generating and sensing system generates and measures e.g., the surface photovoltage (SPV), at the front surface. A series of SPVs are measured and used to derive the minority carrier diffusion length L of the sensor. The carrier diffusion length value is used to calculate the surface recombination velocity at the back surface. The surface recombination velocity value is used to determine the surface concentration of contaminants at the back surface, which may be used to calculate the concentration of contaminants in the fluid medium.

TECHNICAL FIELD

The present invention relates to a new and useful system and method forsensing contamination of a fluid medium, particularly metalcontamination of a fluid medium used in the production of integratedcircuitry in semiconductor wafers.

BACKGROUND OF THE INVENTION

The production of integrated circuits ("ICs") in semiconductor materialsgenerally involves bringing different fluids into direct contact withthe semiconductor materials. For example, in the production of ICs insilicon (Si) and/or gallium arsenide (GaAs) wafers, it is common tobring gases or liquid chemicals into contact with the wafers forpurposes such as: (i) cleaning, (ii) etching, (iii) application ofresists, etc.

It is well known that metal impurities present in the gases or liquidchemicals used in processing semiconductor materials have a profounddetrimental effect on manufacturing yield and the performance ofintegrated circuits. These impurities, present in starting gases orliquid chemicals, or generated by equipment failures, react with thesemiconductor materials to form electron hole recombination centerswhich degrade the semiconductor materials and any integrated circuitsproduced therefrom.

In processing semiconductor materials, metal impurities are difficult tomonitor and control, due to the very low threshold concentrationsrequired for IC yield degradation. The range of surface contamination ofmost interest in the semiconductor industry is between 10⁸ atoms/cm² and10¹² atoms/cm². The problem is even more complex due to the widevariability of the contaminating power of different chemicals used in ICprocessing. For example, NH₃ OH with 0.1 ppm of iron (Fe) will leaveabout 10¹² Fe atoms/cm² on a silicon surface, which is extremelydamaging, while hydrogen fluoride (HF) with the same Fe concentrationwill cause only marginal contamination on the level of 10⁹ Fe atoms/cm².

Thus, as a quality control measure during the production of ICs, it iscommon to measure the contamination of the fluid medium(s) which contactthe semiconductor materials for metal impurities which degrade thequality of the semiconductor material(s). For example, it is common tomeasure the contaminants in the fluid medium(s) before and afterprocessing to ensure that undue contamination has not occurred. Onemethod of measuring the extent of the contamination of the fluids is byspectroscopy. However, this method of testing is not suitable forcontinuous monitoring the fluid because a sample of the fluid must beremoved periodically from the system for testing. Meanwhile, the wafersprocessed by the contaminated fluids before the contamination isdetected become contaminated themselves. These contaminated wafers mayundergo at least partial processing into chips--processing that iswasteful because the wafer is already contaminated.

One common method for determining the contamination of the processedwafers measures a property of the semiconductor known as the minoritycarrier diffusion length "L". The minority carrier diffusion lengthindicates the effective distance that excess carriers diffuse through asemiconductor during their lifetime. Excess carriers in a semiconductortend to redistribute due to a diffusion phenomenon which equalizes thecarrier concentration. The fewer the recombination sites, the fartherthe excess carriers can diffuse before they recombine. In other words,longer measured diffusion lengths correspond to fewer recombinationsites. This diffusion process is controlled by the mobility of theexcess minority carriers "μ" and their lifetime "τ". The diffusionlength L is a parameter combining these two factors, and in the simplestcase has the form: ##EQU1## where k is Boltzman's constant, T is thetemperature in Kelvin, and q is the elementary charge.

As discussed above, metal contaminants in silicon wafers act asrecombination centers which reduce the minority carrier lifetime τ. Bymeasuring the diffusion length L, the concentration of the contaminantsmay then be determined by using the relationship N_(c) ≈Cτ⁻¹ (whereN_(c) is the concentration of heavy metal contaminants, and C is aconstant depending on the individual impurity).

A common, nondestructive technique for measuring the diffusion length Ltakes advantage of the process by which light impinging upon asemiconductor surface may be absorbed and produce excess carriers (holesand electrons) if the energy of the incident photons, "hv", is above thesemiconductor energy band gap "E_(g) ". As a result of thisphotogeneration and diffusion process, a certain number of electron-holepairs reach the proximity of the surface and become separated by theelectric field of the surface-space charge region to produce aphotovoltaic effect refered to as "surface photovoltage" (SPV).Measurement of the surface photovoltage can thus be used for thedetermination of the minority carrier diffusion length L, in turn forthe determination of the lifetime τ, and hence for a determination ofthe concentration of the heavy metal contaminants N_(c).

Some prior techniques for determining the diffusion length from thesurface photovoltage rely on a procedure known as the "ConstantMagnitude Surface Photovoltage" (CMSPV) technique, the principles ofwhich were proposed by Goodman in "A Method for the Measurement of ShortMinority Carrier Diffusion Lengths in Semiconductors," J. Appl. Phys.Vol. 33, p. 2,750, 1961; subsequently adopted as the ASTM standardANSI/ASTM F-391-78 p. 770, 1976 and discussed in U.S. Pat. No.4,337,051.

The characteristic steps of the foregoing techniques are to measure thephotovoltage and the photon flux at several wavelengths (λ₁ . . . λ_(i))(corresponding to photon energies (hv₁ . . . hv_(i))); vary themagnitude of the photovoltage by adjustment of the incident lightintensity or photon flux (φ) to produce a constant photovoltage; measurethe corresponding photon fluxes (φ₁ . . . φ_(i)); and then plot thephoton flux values versus the reciprocal absorption coefficient α⁻¹ ofthe semiconductor sample at the various photon energies. This plot isthen linearly extrapolated to determine the intercept along thereciprocal absorption coefficient axis to obtain the minority carrierdiffusion length L (i.e., L=-α⁻¹ where I=O). Thus, in the CMSPVtechniques, the diffusion length is determined from the correspondingvalues of the photon fluxes (φ₁ . . . φ_(i))) required to maintain theconstant magnitude SPV signal V₁ =V₂ =V₃ . . . .

A more recent technique for determining the diffusion length L isdisclosed in the applicant's U.S. Pat. No. 5,025,145. According to thistechnique, an induced photovoltage is first measured for differentphoton fluxes to assure linearity of photovoltage versus photon-flux.Next, using light with constant photon flux of the value within thelinear photovoltage range, the photovoltage is measured for a series ofselected photon energies and those photovoltage values whichmonotonically increase with the photon energy are plotted as a functionof the reciprocal absorption coefficient corresponding to the givenphoton energies. The minority carrier diffusion length is determined byextrapolation to find the reciprocal absorption coefficient at zerophotovoltage (i.e., L=-α⁻¹ where φ/ΔV=0). The values outside of themonotonical range are rejected from the analysis, which eliminatesinterference from the surface effects and assures an accuratedetermination of the diffusion length. This method determines diffusionlength directly from the surface photovoltage measured in the differentincident photon energies.

Additional background information on sensing metal contamination ofsemiconductor wafers may be found in J. Lagowski, et al., "Non-ContactMapping of Heavy Metal Contamination for Silicon IC Fabrication", Vol.7, Semiconductor Science & Technology, A185-A192 (1992), a copy of whichis attached hereto as Exhibit A and is made part of this disclosure.

Several methods exist to take SPV measurements. One method is thecontact electrode, wherein a semitransparent material such as indium tinoxide (ITO) is placed in contact with the silicon surface beingilluminated. Another method is to capacitively couple the SPV to anelectrode. One specific type of capacitive coupling electrode is thenon-contact electrode. With a non-contact electrode, the dielectric isair, which allows the wafer being tested to remain untouched by theelectrode. The Lagowski, et al. paper, Exhibit A, has a specificdiscussion of capacitive coupling and non-contact sensing of SPVs atpages A187-88.

In both of the SPV techniques discussed above, a semiconductor wafer isused as a sensor, and the surface photovoltage is measured at thesurface that was in direct contact with the fluid medium. In theapplicant's experience, the wafer used in such techniques would have athickness which is considerably greater than its bulk diffusionlength--typically, at least twice as thick.

However, in these thick-wafer SPV methods, the configuration is suchthat the measured parameter, i.e., the minority carrier diffusionlength, is sensitive to the recombination centers in the bulk, but isnot sensitive to surface contaminants. Therefore, to test forcontamination, the contaminants, which typically form on the surface ofthe wafer, are annealed into the wafer at a high temperature in anadditional processing step. For example, a test wafer is included withthe wafers that were being processed. After a various number ofprocessing steps are complete, the process is shut down and the testwafer is annealed and analyzed to determine the amount of contaminationthe batch has sustained. Therefore, these methods of determining theextent of contamination are believed not suitable for constantmonitoring of fluids used in semiconductor processing.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a semiconductor contamination monitoringsystem and method in which a relatively thin wafer is used as a sensor,and photovoltaic measurements (such as SPV measurements) are taken atthe surface of the wafer which is opposite from the surface in contactwith the fluid medium to determine if the wafer evidences an undesirablelevel of contamination in the fluid medium. Thus, the sensor can beemployed in a "continuous" type semiconductor processing system, withthe sensor located in a sensing chamber disposed upstream of asemiconductor processing chamber.

Specifically, according to the invention, a sensor is preferably formedin the shape of a wafer having a front surface and a back surface. Theback surface of the wafer has a very low surface recombination value andis in direct contact with the fluid medium, while the front surface ofthe wafer has a depletion-type surface barrier. The sensor is preferablymade from relatively pure silicon or other semiconductor material whichis reactive with semiconductor contaminants to form recombinationcenters at the back surface and which enables generation and diffusionof electrons and holes therethrough. Further, the wafer has a bulkdiffusion length and a thickness such that the magnitude of surfacephotovoltage developed at the front surface by means of light at apredetermined excitation state is dependent on the concentration ofsemiconductor contaminants at the back surface of the wafer. Accordingto the preferred form of the invention, the wafer has a thickness thatis less than the bulk diffusion length of the wafer.

Additionally, according to the invention, a photovoltaicgenerating/measuring device is provided for: (i) directing light at thepredetermined excitation state at the front surface of the wafer todevelop a photovoltaic effect (e.g., a surface photovoltage), and (ii)measuring the magnitude of the photovoltaic effect (e.g., the surfacephotovoltage) at or near the front surface. The changes in the magnitudeof the photovoltaic effect can then used to determine whether the fluidsample evidences an undue level of semiconductor contaminants.

According to the preferred embodiment, the photovoltaicgenerating/measuring device preferably includes:

(a) a light source for producing a light signal,

(b) a filter for controlling the intensity of the light signal,

(c) a chopper for controlling the wave form of the light signal,

(d) a filter structure for controlling the wave length of the lightsignal;

(e) a pick-up electrode for sensing surface photovoltage developed atthe front surface of the sensor; and

(f) a detection circuit for measuring the magnitude of the surfacephotovoltage sensed by the pick-up electrode.

One preferred type of semiconductor processing system for the inventionincludes a semiconductor processing chamber and a sensing chamberdisposed upstream of the semiconductor processing chamber. The sensingchamber has an inlet for receiving a sample of the fluid medium and anoutlet for directing the sample of the fluid medium into thesemiconductor processing chamber. The sensor wafer forms at least partof the sensing chamber with the back surface of the sensor disposed indirect contact with the fluid medium in the sensing chamber. Adetermination of the fluid contamination on the back surface of thewafer can thus be determined by measuring the surface photovoltage onthe front surface of the wafer. Thus, contamination may be measuredwithout interrupting the system to, e.g., anneal and measure a testwafer, and therefore, the sensor can be used as part of a "continuous"type of semiconductor processing system.

It is therefore one object of this invention to provide a system andmethod for detecting metal contaminants on the surface of asemiconductor wafer.

It is a further object of this invention to provide a system and methodfor continuously testing fluids used during semiconductor waferprocessing for metal contaminants.

Additional objects of the present invention will become further apparentfrom the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a semiconductor processing systemaccording to the present invention;

FIGS. 2A and 2B are schematic illustrations of a portion of thesemiconductor processing system of FIG. 1 showing one type of sensor andprobe constructed according to the present invention;

FIG. 3A is a schematic illustration of another type of sensor and probestructure for the present invention;

FIG. 3B is an electrical schematic drawing of the capacitive coupling ofthe sensor and probe structure of FIG. 3A;

FIG. 4 is a schematic illustration of a third type of sensor and probestructure for the present invention;

FIG. 5 is a schematic illustration of an additional aspect of thepresent invention wherein a portion of the sensor and probe areencapsulated;

FIG. 6A is a graphical representation of the effect of the sensorthickness on the measured diffusion length value for three differentvalues of surface recombination velocity at the back surface of thesensor wafer;

FIG. 6B is a graphical representation of the effect of the back surfacerecombination velocity and the corresponding surface metal density onthe measured diffusion length value for sensors with differentthickness; and

FIG. 6C is a graphical representation of the effect of the back surfacerecombination velocity and the corresponding surface metal density onthe measured diffusion length value for sensors with different diffusionlengths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As discussed above, a preferred application of the present inventionrelates to a semiconductor contamination monitoring system using an SPVsilicon sensor. The sensor is preferably formed from a relatively thin,substantially pure silicon wafer, with a thickness (T) which issubstantially less than its bulk diffusion length (L.sup.∞). The sensoris incorporated into a sensing chamber of the monitoring system in whicha fluid medium is being tested for metal contamination. The back surfaceof the silicon wafer is in direct contact with the fluid medium in thechamber. The metal contaminants deposited on the back surface of thesilicon sensor act as electron-hole recombination centers. Therefore,the metal contaminants change the surface photovoltage measured on theopposite (i.e., front) surface of the silicon wafer. Using SPV diffusionlength measurements, the surface recombination velocity can bedetermined and converted into the surface concentration of depositedmetals on the back surface of the silicon wafer.

Principle of the SPV Sensor

The underlying principle behind this invention is the applicant'srecognition that for thin sensor wafers (i.e., with a thickness (T) lessthan bulk diffusion length (L.sup.∞)), the magnitude of the photovoltaiceffect at the front surface of the wafer is sensitive to surfacerecombination at the back surface of the wafer. For example, twoparameters can be measured, the minority carrier diffusion length "L"and the surface photovoltage "ΔV", which can be used to determine theback surface recombination velocity "S_(b) ". Other photovoltaic effectscan also be used to determine ΔV, for example, P-N junction photovoltageor semi-conductor metal photovoltage. Accordingly, the terms"photovoltaic" and "photovoltaic effect" as used herein is intended toencompass these and other photovoltaic effects. Measuring theseparameters enables surface contamination on the back surface of thewafer to be monitored via the surface photovoltage measurementsperformed on the opposite (i.e., front) surface.

According to Frankl and Ulmer (D. R. Frankl and E. A. Ulmer, Theory ofthe Small-Signal Photovoltage at Semiconductor Surfaces, SurfaceScience, 6 (1966), p. 115), the small signal photovoltage generatedunder low excitation level can be expressed as: ##EQU2## with:

    C=(S.sub.f S.sub.b L/D+D/L) sinh(T/L)+(S.sub.f +S.sub.b) cosh(T/L);

where:

T is the wafer thickness;

φ_(eff) is the photon flux entering the sensor (i.e., corrected forlight loss due to reflectivity of front surface);

f(V₀,E_(f)) is a known function of the initial surface potential barrierV_(o), and the Fermi energy in the semiconductor E_(f) ;

D is the minority carrier diffusivity;

α is the absorption coefficient;

α⁻¹ is the light penetration depth;

αT>>1, i.e., the entire light is absorbed by the wafer;

L is the minority carrier diffusion length L=(Dτ)^(1/2) where γ is theminority carrier lifetime;

(we denote this value also as L.sup.∞ because it is the value measuredby the SPV method when the wafer thickness increases); and

S_(f) and S_(b) are the surface recombination velocities on the frontsurface and back surface, respectively.

When used in the thick-wafer SPV method as described previously, ΔV ismeasured for a series of α values and analyzed in a way enabling adetermination of L. The measuring conditions are such that the waferthickness T is at least twice as long as the diffusion length L.

When this is the case, sinh(T/L)≃cosh(T/L)≃e^(T/L) /2 and Equation (1)reduces to a simple form: ##EQU3## Note that in the thick-wafer case,the magnitude of the surface photovoltage ΔV depends on the surfacerecombination velocity S_(f) on the front surface, but is independent ofthe surface recombination velocity of S_(b) on the back surface.Equation (2) can be rewritten as: ##EQU4## which is a basis for astandard SPV plot of: ##EQU5## yielding the L value from the α⁻¹ at theintercept φ/ΔV=0.

According to the underlying principles of the present invention, whenthe condition T/L≧2 is not satisfied, then the photovoltage becomes afunction of the back surface recombination S_(b) as expressed byEquation (1). Under constant photon flux conditions, φ_(eff) =constantfor all α values used. The photovoltage normalized to the value at aparticular wavelength λ, and thus also a particular absorptioncoefficient α, becomes

    ΔV*=ΔV.sup.(α) /ΔV(α.sub.1)

which can be expressed as: ##EQU6##

This normalized photovoltage depends on S_(b) but not on S_(f). Thus,according to the principles of the present invention, the spectralmeasurements of ΔV*(α) can be used for determination of S_(b) providingthat the other wafer parameters in equation (4) are known, such as T andL.

According to a preferred form of the applicant's invention, the sensorfor measuring the surface recombination velocity S_(b) is made of asilicon wafer with a predetermined L value. The ratio T/L is preselectedto achieve maximum sensor sensitivity and a suitable range of S_(b)values, which should correspond to a typical range of surface metalrecombination--i.e., from about 1 cm/s to 10⁴ cm/s in silicon.

In determining S_(b) from equation (4), one can use various methods forfitting an experimental curve ΔV*(α) with S_(b) as a parameter. Themethod of the present invention is an extension of the basic SPV methodsdesigned for thicker wafers, in that it is also based on determining theeffective L value to be denoted L_(m) for thin wafers. Moreover, it isbased further on the observation that even in a range of thin waferssuch that a simplifying condition T/L>2 is not satisfied, the plot of##EQU7## can still be approximated in a reasonable way by a lineardependence ##EQU8## however the L_(m) ⁻¹ value is no longer equal to theminority carrier diffusion length L, but it depends on the T/L ratio andon the S_(b) value.

The method of the present invention, therefore, proposes to use theL_(m) value as a sensitive and convenient measure of S_(b) for waferswhere the thickness T is less than the bulk diffusion length L.sup.∞.The method of the present invention can be realized using the types ofSPV sensors illustrated in FIGS. 2A, 2B, 3A, 4 and 5. As will bedescribed herein, all these sensors have an exposed back surface whichcontacts the contaminated fluid in the monitoring system.

SPV measurements are done from the front, illuminated surface using thesame procedure and apparatus as in a "linear, constant photon flux"method such as described in applicants' U.S. Pat. No. 5,025,145. Theback surface recombination velocity S_(b) can be determined by fittingΔV* to equation 4, or alternatively using L_(m) values andcomputer-stored calibration curves for a given sensor S_(b) vs. L_(m)(curves like those shown in FIGS. 6B and 6C). Knowing T and L for thesensor and calibration curve therefore instantaneously yields the S_(b)value and the corresponding metal contamination.

As indicated above, the concentration of contaminants can be determinedfrom the back surface recombination velocity S_(b). For low excitationlimits (i.e., the low intensity of the light-generating SPV signal), theS_(b) value is directly proportional to the surface density of therecombination centers, N_(s) :

    S.sub.b =N.sub.s •ν.sub.th •σ         (6)

where ν_(th) is the thermal velocity of minority carriers (about 10⁷cm/s in silicon at room temperature) and σ is the capture cross sections(typically in the 10⁻¹⁵ cm² range). Rough estimation based on the aboveexpression gives N_(s).sup.˜ 10⁸ •S_(b), and such an N_(s) is within asurface concentration range (10⁸ /cm² -10¹² /cm²) believed to besignificant for silicon microelectronics applications. Thus, since themagnitude of the surface photovoltage ΔV is dependent on the backsurface recombination velocity S_(b), and S_(b) is in turn dependentupon the surface density N_(s) of the recombination centers, and finallysince the surface density N_(s) of the recombination centers isdependent upon the concentration of contaminants in the fluid medium,then the magnitude of the surface photovoltage at the front surface isdependent upon the concentration of contaminants in the fluid medium.Hence, the surface photovoltage ΔV can be measured at the front surfaceand used to determine whether undue contamination of the fluid samplehas occurred at the back surface.

FIGS. 6A-6C validate the principle of the thin-wafer sensorconfiguration discussed above. Most of the plotted values werecalculated using highly accurate computer models of the thin-wafersensor. Additionally, several points were verified by actual measuredvalues.

More particularly, FIG. 6A shows the effect of thinning the sensor, witha bulk diffusion length L.sup.∞ =700 μm, on the measured diffusionlength value, L_(m), for three different values of surface recombinationvelocity at the back surface, S_(b). Note that for low S_(b) values,L_(m) increases above the bulk value, and for very high S_(b) values,L_(m) decreases below that of L.sup.∞. The difference ΔL_(m) =L_(m) (lowS_(b))-L_(m) (high S_(b)) increases with decreasing sensor thickness,thereby increasing the sensor range.

Further, FIG. 6B is a plot of the diffusion length, L_(m), versus theback surface recombination velocity, S_(b), and a corresponding surfacemetal density for sensors with different thickness. For all sensors,L.sup.∞ =1,000 μm. Note that the sensor range increases with decreasingthickness.

Finally, FIG. 6C is a plot of the diffusion length, L_(m), versus theback surface recombination velocity, S_(b), and corresponding surfacemetal density for 600 μm thick sensors with both diffusion lengthL₁.sup.∞ =2,000 μm and L₂.sup.∞ =1,000 μm. Note that the sensor rangeincreases with increasing L.sup.∞ value.

The measured values in FIGS. 6A-6C were measured using a CMS III devicesuch as disclosed in the applicant's U.S. Pat. No. 5,025,145 on a wafermade of p-type silicon doped with boron to a level of 2×10¹⁵ cm⁻³. Thelevel of surface contamination of copper was known and was applied tothe back surface of the wafer from either a buffered HF solution or anH₂ O₂ solution. It is believed that the actual contaminant used is notimportant because virtually any metal will provide recombination centersin semiconductor materials. Therefore, the shape of the curves shown inFIGS. 6A-6C are typical of what would be seen if any metal contaminantwas used.

Semiconductor Processing System

A semiconductor processing system with an SPV sensor constructedaccording to the invention is schematically illustrated in FIG. 1, whilea more detailed illustration of a sensor for the processing system isindicated generally at 10 in FIGS. 2A and 2B. The sensor 10 comprises ap-type silicon wafer, polished and etched on both sides, which ismounted by e.g., teflon O-rings, to the bottom of a teflon fluid testingchamber 12. The silicon used for the sensor must be of very high purity,with a bulk diffusion length of about 1,000 μm or higher and a thicknessof about 400 μm.

Surface preparation of the sensor wafer should satisfy two conditions:(1) a depletion type surface barrier on the front surface 13 (e.g., aP-N junction or any other rectifying contact suitable for generation ofa photovoltage); and (2) a very low surface recombination on the backsurface 15 facing and in direct contact with fluid 16 in the testingchamber 12. The low original surface recombination value on the backsurface 15 is necessary to enhance the detection sensitivity forcontaminants deposited from the liquid.

The fluid testing chamber 12 of the semiconductor processing systemincludes a fluid inlet 22 and a fluid outlet 24. The fluid inlet 22 isconnected to a fluid reservoir 26. The fluid outlet 24 is connected to afluid pump 27. A semiconductor wafer processing station 28 is connectedbetween the fluid pump 27 and the fluid reservoir 26 for re-use. Thefluid pump 27 directs the fluid into the wafer processing station 28,and fluid from the processing station 28 is directed back to the fluidreservoir 26. Alternatively, the fluid from the wafer processing station28 can be connected to a discharge reservoir (not shown) and not bere-used. In any case, in the processing system of FIG. 1, the fluidtesting chamber 12 is "in line" with, and upstream of, the waferprocessing station 28.

Nevertheless, it should be apparent to those skilled in the art that theabove-described processing system is only exemplary in nature, and othertypes of processing systems can also be used with the sensor of thepresent invention. In other words, the sensor of the present inventionhas wide adaptability to many types of commercially-used processingsystems for manufacturing integrated circuits.

The sensor 10 forms part of the fluid testing chamber 12 and is attachedthereto (e.g., sealed around the edges) in any conventional manner. Theback surface 15 of the sensor 10 is in contact with the fluid in thetesting chamber 12 as the fluid passes on the way to the waferprocessing station 28. A photovoltaic generating/measuring device,indicated generally at 32, is associated with the front surface 13 ofthe sensor 10, as will be described more fully hereinafter.

As discussed in applicant's U.S. Pat. No. 5,025,145, thegenerator/measuring device 32 preferably employs a quartz halogen bulb34 held in a source housing 36. Radiation produced by the bulb isfocused and passed first through a rotating chopper 38 held within thesource housing 36. The rotating chopper is operated at a selectedfrequency in the range between 5 Hz-100 Hz for direct contact SPVmeasurements, and between 500 Hz-600 Hz for capacitive-coupled SPVmeasurements. Following the chopper 38 is the light intensity attenuator39, an iris diaphragm 40 and a graduated variable neutral density filter42, held in an assembly housing 44. The attenuator 39 is adjustable forincreasing or decreasing light intensity in the linearity measurementprocedure to be described below.

After passing through the filters, the attenuated beam is focused onto afirst glass fiber optical cable 46 which is coupled by e.g., adhesiveepoxy, to the filter assembly 48 by means of, e.g., cable mounts. Theoptical cable 46 brings radiation from the bulb 34, modulated by thechopper 38 to a filter wheel 50 which may be used for wavelengthselection. Each filter in the filter wheel 50 consists of a narrow bandpass filter and a customized set of neutral density filters, the formerassuring substantially monochromatic light and the latter being used toachieve a substantially constant photon flux φ_(eff). =const.

After passing through a selected filter of the filter wheel 50, themonochromatic beam is coupled to a fiber optical cable 52 which directsthe radiation toward the front surface 13 of the sensor 10 (see FIG. 2).The use of the fiber optical cables in this preferred embodimentprovides a particular advantage of the apparatus in that the system neednot be entirely shaded from ambient light which might otherwise causemeasurement errors or increased noise in conventional open opticdesigns.

The light incident upon the sensor 10 generates a photovoltage which isdetected by a probe, indicated generally at 54, which is connected to alock-in amplifier 56 of an electronic system via a lead 60. The probe 54is also electrically connected to the lock-in amplifier 56 via referenceground lead 82. The electronic system further includes a control alonglead 58 for the light source chopper 38 which provides a referencesignal for the lock-in amplifier 56.

The filter wheel 50 includes eight filters (labeled 1-8 in FIG. 1) whichmay be selectively positioned in the radiation path of the optical cable52 by rotating filter wheel 50. Two filters for use in the linearitycheck, for example, filters 1 and 2, transmit white light and have anintensity ratio of I₁ /I₂ =2.00±0.05. Filters 3-8 are composed of thenarrow band (half width, e.g., ≦0.01 ev) pass interference filters thattransmit monochromatic radiation and the customized neutral densityfilters which assure a similar photon flux within ±2% for use inminority carrier diffusion length measurements. An example of a set offilter characteristics for the measurement of diffusion length L insilicon wafers are given in Table I below:

    ______________________________________                                                     Photon  Output Effective                                         Wheel        Energy  Photon Flux                                              Position     (in eV) (arb. units)                                             ______________________________________                                        3            1.210   1.00                                                     4            1.240   0.98                                                     5            1.269   1.02                                                     6            1.303   1.00                                                     7            1.340   0.99                                                     8            1.380   1.01                                                     ______________________________________                                    

The probe 54 accepts the optical cable 52 and directs the radiation tothe sensor front surface 13 and detects the induced photovoltage. Theprobe 54 may be supported on a vertical positioner which allows raisingand lowering the probe relative to the front surface 13 of the sensor10.

The probe 54 includes a jacket 64 constructed from a material such as adark plastic which is not transparent to light and comprises a firstaperture 66 for receiving the fiber optic cable 52 which may be attachedsealably or via common couplers, and a second aperture 68 disposedopposite the first aperture for vertical direction of the providedradiation. The second aperture 68 defines the photovoltage probing areaon the wafer which is approximately equal to the cross-sectional area ofthe second aperture. Preferably, between the first and second aperturesand beyond the point at which the fiber optic cable terminates in theprobe jacket 64, one or more lens 70 are provided for forming a uniformbeam of radiation through the second aperture 68. About the secondaperture 68, the jacket 64 forms a base region 72 which is ofsubstantially greater outside diameter than the aperture 68, forexample, the outer diameter of the base region 72 is preferably about 25mm, while the diameter of the aperture 68 is about 2 mm.

The probe 54 is provided with a semi-transparent pickup electrodeassembly 74 and a reference ground assembly 76, which will be explainedin further detail below. The electrode assembly, shown generally at 74,and the reference ground assembly shown generally at 76 are anchored tothe base region 72 of the jacket 64. The electrode assembly 74 willtypically comprise a roughly circular area. The reference groundassembly 76 may comprise a single, roughly circular area or an annularring or rectangular shape around the electrode assembly 74.

The base region 72 has a light shield 76, which is substantially flushagainst the front surface 13 of the sensor 10. The light shield 76surrounds the electrode assembly 74 and may be annular ring-shaped orrectangular in shape, for example.

According to one embodiment of the present invention, shown in FIG. 2B,the electrode assembly 74 includes a semi-transparent flexible foilphotovoltage pickup electrode 84, made of, for example, indium-tinoxide, covering a semi-transparent block 86, made of, for example,quartz. The electrode assembly has associated with it an electrode lead60, which connects directly to the lock-in amplifier 56.

The reference ground assembly includes a reference ground electrode 94,which can also be made of e.g., indium-tin oxide, and has associatedwith it a reference ground lead 82, which electrically communicates thereference signal from the lock-in amplifier 56.

Light from the fiber optic cable 52 passes through lens 70, thesemi-transparent block 86, and the semi-transparent flexible foilphotovoltage pickup electrode 84. The light impinges upon the frontsurface 13 of the sensor 10 and creates a photovoltaic effect dependentupon the concentration of contaminants in contact with the back surface15 of the sensor. For example, the light can create a surfacephotovoltage at the front surface in the absence of any junctions on thewafer; or can create a photovoltage near the surface of the wafer ifother photovoltaic techniques are used, e.g., P-N junction photovoltageor semi-conductor photovoltage.

In any case, the electrode pickup 74 senses the photovoltage and relaysthis information to lock-in amplifier 56, which then provides an outputsignal to computer 79 which digitizes the signal and provides anindication of the contaminants in the fluid.

The computer 79 then uses a series of ΔV measurements to calculate theminority carrier diffusion length, L_(m). L_(m) is used to calculate theback surface recombination velocity, S_(b), which is, in turn, used tocalculate the surface concentration of contaminants, N_(s).

The light shield 73 of the base 72 performs two functions. First, straybackground light is blocked from the surface area interrogated becausethe light shield 73 is i) flush against the front surface 13 of thewafer 10, ii) surrounds the electrode assembly 74, and iii) the frontsurface 13 of the wafer is larger than the wafer area imaged by the beamand measured by the pickup electrode. This design represents asignificant improvement over previous surface photovoltage probes bypermitting the use of the probe in the presence of ambient light andthereby avoiding the inconvenience of shading the entire wafer, whilestill reducing noise levels in the measurement.

Referring now to FIG. 3A, a preferred embodiment of the probe 54 and thesensor 10 is illustrated. In FIG. 3A, a MOS-type sensor 100 is shownformed from p-type silicon. In the MOS-type sensor 100, the electrodeassembly 54 and the reference ground assembly 76 of the probe 74capacitively couple the SPV. The sensor 100 has a front surface 101 anda back surface 102. The electrode assembly 74 of the probe 54 includes atransparent dielectric 104 deposited on the front surface 101 of thesensor 100. The semi-transparent dielectric 104 may be a typicaldielectric such as SiO₂ or Si₃ N₄, and may be grown or depositeddirectly onto the front surface 101 of the sensor 100.

A semi-transparent pickup electrode 106 is deposited on the outersurface of the semi-transparent dielectric 104. The semi-transparentpickup electrode 106 may be made of semi-transparent metal (about 200 Åthick) evaporated onto the semi-transparent dielectric 104. Thesemi-transparent pickup electrode 106 should be smaller than the frontsurface 101 of the sensor 102, and should be transparent enough to allowlight from the second aperture 68 of probe 54 to pass through to thefront surface 101, allowing the light to generate electron hole pairs,and therefore, a surface photovoltage.

The semi-transparent pickup electrode 106 has signal lead 60 connectedat a pickup contact 110. The pickup lead 60 connects to a high inputresistance preamplifier and the lock-in amplifier 56 (FIG. 1).

In this preferred embodiment, the reference ground assembly 76 of theprobe 54 is also attached to the front surface 101 of the sensor 100.The reference ground electrode 116 may be a metal deposited directly onsilicon or on the dielectric film.

The reference ground electrode 116 has associated with it a referenceground lead 82 which is communicated with a preamplifier and the lock-inamplifier.

Referring now to FIG. 3B, an electrical schematic view of the capacitivecoupling of the MOS-type sensor 100 of FIG. 3A is shown. The sensor 100and the semi-transparent pickup electrode 106 form a capacitor C_(PE)130 with the semi-transparent dielectric 104 acting as the dielectricmedium. Also, the sensor 100 and the reference ground electrode 116 forma capacitor C_(RG) 132. The reference ground lead 82 and the pickup lead60 and the input resistor of the preamplifier complete the circuit shownin FIG. 3B.

Additional modifications and variations of the present invention arepossible.

For example, referring now to FIG. 4, a further embodiment of a sensoris shown with a P-N type junction. According to this embodiment, asensor 200 is formed of p-type silicon. The sensor 200 has a frontsurface 201 and a back surface 202. The electrode assembly 74 of theprobe 54 includes a pickup N⁺ region 204 grown directly onto or dopedinto the front surface 201 of the sensor 200, which is sufficiently thinto be transparent to an SPV generating light beam. The pickup N⁺ region204 forms a P-N junction on which the photovoltage is generated. Thepickup N⁺ region 204 has a signal pickup lead 60 associated with itwhich connects to the pickup N⁺ region 204 at a pickup contact 210. Thepickup lead 60 connects directly to the preamplifier and the lock-inamplifier 56 (FIG. 1). Again, as described previously, the pickup N⁺region 204 should be smaller than the front surface 201 of the sensor200 and must allow light from the second aperture 68 of the probe 54 topass through to the sensor 200 below, allowing the light to generateelectron whole pairs and therefore a photovoltage.

The reference ground assembly 76 for this embodiment has a referenceground ohmic contact (evaporated AL) to p-type base region at thesensor.

Again, as described with respect to the previous embodiment, the P-Njunction-type sensor 200 has a thickness T which is less than the bulkdiffusion length, L.sup.∞. Further, the back surface 202 of the sensor200 is in direct contact with the fluid 16 being measured by the sensor.

Referring now to FIG. 5, an encapsulated miniature sensor 300 is shownmade according to another embodiment of the present invention. Thissensor could be manufactured in an integrated miniaturized form and issuitable for emersion into the liquid being monitored. The encapsulatedminiature sensor could be made like any of the types of systemspreviously described, such as the MOS-type sensor 100, the P-Njunction-type sensor 200, or the flexible foil electrode sensor 10. Theencapsulated miniature sensor 300 has a front surface 301 which isilluminated and a back surface 302 which is exposed to the fluid 16being monitored.

The sensor 300 with the associated pickup electrode assembly 304 and thereference ground assembly 314 is encapsulated in an encapsulation medium322, which covers most of the encapsulated miniature sensor 300, exceptfor a portion of the back surface 301 corresponding to the illuminatedportion of the front surface 301. The exposed portion of the backsurface 301 is in contact with the fluid being monitored. Theencapsulation medium 322 should be formed from a material which isresistant to damage which may be caused by the harsh fluids used insemiconductor processing (e.g., teflon). Moreover, in this embodiment, aportion of the encapsulation medium 322 can form the light shield 73.

The examples shown in this specification refer to systems having sensorsmade of p-type silicon and several regions are denoted as N⁺ regions.P-type silicon sensors are preferred because the bulk minority carrierdiffusion length L.sup.∞ in p-type silicon is longer than that in n-typesilicon. However, for some applications, an n-type sensor may also besuitable.

Other modifications of the present invention are also possible whenconsidered in light of the above teachings. It is therefore understoodthat the scope of the present invention is not to be limited to thedetails disclosed herein, and may be practiced otherwise than asspecifically described, and is intended only to be limited by the claimsappended hereto.

What is claimed is:
 1. A system for measuring contamination of a fluidmedium, comprising:(a) a sensor having a back surface and a frontsurface, said back surface being in direct contact with a sample of thefluid medium, said sensor being made of material which is reactive withsemiconductor contaminants to form recombination centers at said backsurface and to enable diffusion of electrons and holes therethrough,said sensor having a bulk diffusion length and a thickness such that themagnitude of a photovoltaic effect developed at or near said frontsurface by means of light at a predetermined excitation state isdependent on the concentration of recombination centers on the backsurface; and (b) photovoltaic generating and measuring means for (i)directing light at said predetermined excitation state at said frontsurface to develop the photovoltaic effect at or near said front surfaceand (ii) measuring the magnitude of the photovoltaic effect at or nearsaid front surface.
 2. A system as defined in claim 1 wherein thethickness of said sensor is no greater than its bulk diffusion length.3. A system as defined in claim 2 wherein said sensor is formed ofsubstantially pure silicon.
 4. A system as defined in claim 3 whereinsaid sensor has the configuration of a thin wafer, said back surface hasa surface configuration which has a very low surface recombination valueand said front surface has a depletion type surface barrier forgenerating the photovoltaic effect.
 5. A system as defined in claim 4,wherein said front surface has a configuration which generates a surfacephotovoltage.
 6. A system as defined in any of claims 1-5, furtherincluding a semiconductor processing chamber and a sensing chamberdisposed upstream of said semiconductor processing chamber, said sensingchamber having an inlet for receiving a sample of said fluid medium andan outlet for directing said sample of said fluid medium into saidsemiconductor processing chamber, said sensor forming at least part ofsaid sensing chamber with said back surface of said sensor disposed fordirect contact with a fluid medium located in said sensing chamber.
 7. Asystem as defined in claim 1 wherein said photovoltaic generating andmeasuring means comprises;(a) a light source for producing a lightsignal, (b) means for controlling the intensity of said light signal,(c) means for controlling the wave form of said light signal, (d) meansfor controlling the wavelength of said light signal; (e) a probe forsensing the photovoltaic effect developed at or near said front surface;and (f) a detection circuit for measuring the magnitude of thephotovoltaic effect sensed by said probe.
 8. A system as defined inclaims 1 or 7, wherein said probe includes:(i) an electrode assemblyhaving an associated electrode lead, said electrode lead beingelectrically connected to said photovoltaic generating and measuringmeans, and said electrode assembly couples surface photovoltage fromsaid front surface to said electrode lead; and (ii) a reference groundassembly having an associated reference ground lead, said referenceground lead being electrically connected to said photovoltaic generatingand measuring means.
 9. A system as defined in claim 8 wherein saidelectrode assembly comprises a semi-transparent dielectric attached tosaid front surface of said sensor, and a semi-transparent pickupelectrode substantially the same size as said semi-transparentdielectric and attached to said semi-transparent dielectric at the sideopposite the side attached to said front surface; andwherein saidreference ground assembly comprises a reference ground dielectricattached to said front surface of said sensor, and a reference groundelectrode substantially the same size as said reference grounddielectric and attached to said reference ground dielectric at the sideopposite the side attached to said front surface.
 10. A system asdefined in claim 9, wherein said semi-transparent dielectric and saidreference ground dielectric comprise SiO₂ or Si₃ N₄ material grown ontosaid front surface.
 11. A system as defined in claim 9 wherein saidsemi-transparent pickup electrode and said reference ground electrodecomprise a thin coating of metal deposited onto said semi-transparentdielectric and said reference ground dielectric.
 12. A system as definedin claim 11, wherein said thin coating of metal on said semi-transparentdielectric and said reference ground dielectric and said referenceground dielectric is about 200 Å thick.
 13. A system as defined in claim8 wherein said electrode assembly comprises an N⁺ region formed at thefront surface of said sensor.
 14. A system as defined in claim 8 whereinsaid electrode assembly comprises an indium tin oxide electrode.
 15. Asystem as defined in claim 8 further including:(i) an optical cable fortransmitting light at said predetermined excitation state from saidphotovoltaic generating and measuring means through said electrodeassembly to said front surface; (ii) an optical coupler, wherein saidoptical coupler optically connects said optical cable to said electrodeassembly; and (iii) an encapsulation medium, said encapsulation mediumat least partially encapsulating said front surface, said opticalcoupler, said electrode assembly, and said reference ground assembly.16. A system as defined in claim 15, wherein said probe includes a lightshield supported proximate said front surface of said sensor andsurrounding said electrode assembly to shield the front surface fromambient light.
 17. A method for producing integrated circuitry in asemiconductor material, comprising the steps of:(a) providing a fluidmedium for use in semiconductor processing during production ofintegrated circuitry in a semiconductor material; (b) providing a sensorcomprising a mass of sensing material having front and back surfaces,said mass of material being reactive with semiconductor contaminants inthe fluid medium to form recombination centers at said back surface andto enable diffusion of electrons and holes therethrough, said sensorhaving a bulk diffusion length and a thickness which causes themagnitude of a photovoltaic effect developed at or near said frontsurface by means of light at a predetermined excitation state to bedependent on the concentration of semiconductor contaminants in thefluid medium, (c) flowing a sample of the fluid medium into directcontact with said back surface of said mass of sensing material, (d)developing the photovoltaic effect at or near said front surface bymeans of light in said predetermined excitation state and measuring themagnitude of the photovoltaic effect developed at or near said frontsurface of said mass of sensing material to determine if the sample ofthe fluid medium has a predetermined concentration of semiconductorcontaminants, and (e) if the sample of the fluid medium has less thanthe predetermined concentration of semiconductor contaminants, using thefluid medium to flow into contact with the semiconductor material aspart of the production of an integrated circuit in the semiconductormaterial.
 18. A sensor structure for use in determining if a sample of afluid medium has semiconductor contaminants therein, said sensorstructure comprising:(a) a substantially flat wafer formed ofsemiconductor material; (b) said wafer having front and back surfaces;(c) said wafer being reactive with semiconductor contaminants in directcontact with its back surface to form recombination centers at said backsurface; and (d) said wafer having a bulk diffusion length and athickness such that a photovoltaic effect developed at or near saidfront surface by means of light in a predetermined excitation state isdependent upon the density of recombination centers which form at saidback surface of said wafer due to semiconductor contaminants in thesample of the fluid medium.
 19. A sensor structure as defined in claim18 wherein the thickness of said wafer is no greater than its bulkdiffusion length.
 20. A sensor structure as defined in claim 19 whereinsaid wafer is formed of substantially pure silicon.
 21. A sensorstructure as defined in claim 20 wherein said back surface of said waferhas a surface configuration which has a very low surface recombinationvalue and said front surface has a depletion type surface barrier.
 22. Asensor structure as defined in claim 18 wherein said sensor structurefurther includes;(a) a light source for producing a light signal, (b)means for controlling the intensity of said light signal, (c) means forcontrolling the wave form of said light signal, (d) means forcontrolling the wavelength of said light signal; (e) a probe for sensingthe photovoltaic effect developed at or near said front surface of thewafer; and (f) a detection circuit for measuring the magnitude of thephotovoltaic effect sensed by said probe.
 23. A sensor structure asdefined in claim 22, wherein said probe includes:(i) an electrodeassembly having an associated electrode lead, said electrode lead beingelectrically connected to said sensor structure, and said electrodeassembly couples surface photovoltage from said front surface to saidelectrode lead; and (ii) a reference ground assembly having anassociated reference ground lead, said reference ground lead beingelectrically connected to said sensor structure.
 24. A sensor structureas defined in claim 23 wherein said electrode assembly comprises asemi-transparent dielectric attached to said front surface of saidwafer, and a semi-transparent pickup electrode substantially the samesize as said semi-transparent dielectric and attached to saidsemi-transparent dielectric at the side opposite the side attached tosaid front surface; andwherein said reference ground assembly comprisesa reference ground dielectric attached to said front surface of saidwafer, and a reference ground electrode substantially the same size assaid reference ground dielectric and attached to said reference grounddielectric at the side opposite the side attached to said front surface.25. A sensor structure as defined in claim 24, wherein saidsemi-transparent dielectric and said reference ground dielectriccomprise SiO₂ or Si₃ N₄ material grown onto said front surface.
 26. Asensor structure as defined in claim 24 wherein said semi-transparentpickup electrode and said reference ground electrode comprise a thincoating of metal deposited onto said semi-transparent dielectric andsaid reference ground dielectric.
 27. A sensor structure as defined inclaim 26, wherein said thin coating of metal on said semi-transparentdielectric and said reference ground dielectric and said referenceground dielectric is about 200 Å thick.
 28. A sensor structure asdefined in claim 23 wherein said electrode assembly comprises an N⁺region formed at the front surface of said wafer.
 29. A sensor structureas defined in claim 23 wherein said electrode assembly comprises anindium tin oxide electrode.
 30. A sensor structure as defined in claim23 further including:(i) an optical cable for transmitting light at saidpredetermined excitation state from said sensor structure through saidelectrode assembly to said front surface; (ii) an optical coupler,wherein said optical coupler optically connects said optical cable tosaid electrode assembly; and (iii) an encapsulation medium, saidencapsulation medium at least partially encapsulating said frontsurface, said optical coupler, said electrode assembly, and saidreference ground assembly.
 31. A sensor structure as defined in claim30, wherein said probe includes a light shield supported proximate saidfront surface of said wafer and surrounding said electrode assembly toshield the front surface from ambient light.
 32. A sensor structure foruse in determining if a sample of a fluid medium has semiconductorcontaminants therein, said sensor structure comprising:(a) asubstantially flat wafer formed of semiconductor material; (b) saidwafer having front and back surfaces; (c) said wafer being reactive withsemiconductor contaminants in direct contact with its back surface toform recombination centers at said back surface; (d) said wafer having abulk diffusion length and a thickness such that a photovoltaic effectdeveloped at or near said front surface by means of light in apredetermined excitation state is dependent upon the density ofrecombination centers which form at said back surface of said wafer dueto semiconductor contaminants in the sample of the fluid medium; and (e)wherein said back surface of said wafer has a surface configurationwhich has a very low surface recombination value and said front surfacehas a depletion type surface barrier.
 33. A sensor structure for use indetermining if a sample of a fluid medium has semiconductor contaminantstherein, said sensor structure comprising:(a) a substantially flat waferformed of semiconductor material, said wafer having front and backsurfaces; (b) a semi-transparent dielectric attached to said frontsurface of said wafer, and a semi-transparent pickup electrodesubstantially the same size as said semi-transparent dielectric andattached to said semi-transparent dielectric at the side opposite theside attached to said front surface; (c) a reference ground dielectricattached to said front surface of said wafer, and a reference groundelectrode substantially the same size as said reference grounddielectric and attached to said reference ground dielectric at the sideopposite the side attached to said front surface; (d) said wafer beingreactive with semiconductor contaminants in direct contact with its backsurface to form recombination centers at said back surface; and (e) saidwafer having a bulk diffusion length and a thickness such that aphotovoltaic effect developed at or near said front surface by means oflight in a predetermined excitation state is dependent upon the densityof recombination centers which form at said back surface of said waferdue to semiconductor contaminants in the sample of the fluid medium.